The latest video seems to indicate that Cass doesn't know who you are and suspects that you may even be behind the explosion when she arrives in the aftermath. If you were important than she would probably have an idea about who you are. I'm guessing you are just a relative nobody. Maybe, if it does happen at the places the ashes are, you live in or near the village of Haven (it's been 10+ years so even if the Warden killed everyone its likely there are new people there, especially with a holy site nearby) and are caught in the blast.
It makes some sense for you to be from there in my random speculation, there could be a Dalish clan traveling nearby at the time since they do that, travel around (maybe even for the meeting) and the other side of the mountains from the village is the Dales so crossing over the mountains could bring them past the village. You're a surface dwarf if my memory serves and the town and temple is relatively close to Orzammar so you may have moved there from the dwarf city when you left for whatever reason. As a Qunari if your parents wanted to get as far from the Qunari as possible you couldn't do much better than a small village in the mountains between Fereldan and Orlais. And a human can just be a villager, and a human mage could just be from the village and went home when the Circle fell apart.
Maybe, whoever you are, you're a village leader of some kind. Even as a dwarf or Qunari you could be important there for any number of reasons. Perhaps as a Qunari your family drove off some bandits that had plagued the village and so was accepted even though they were Qunari. Maybe the dwarf's family helped establish a strong trade house to accommodate all the pilgrims going to the shrine and boosted the village economy thus becoming important. The human could be the son or daughter of the person who rebuilt the village after the events of the fifth blight caused its destruction. The Dalish could be there for the meeting or they may have set up there for some other reason like how Merrile's clan set up near Kirkwall for 10 years, perhaps to study the shrine along side the chantry or to look for other ancient holy sites the elves may have built nearby, and so after all the time in close proximity there has been some trade and a bond has developed between the village and the Dalish and they respect each other.
This gives us a personal lose in the explosion too, if our home is destroyed it may spur us towards getting those responsible more than just, "doing what's right." It gives us another possible motive for someone to lead the Inquisition besides power (I want to lead a powerful group) and honor (saving the world), we can set out for revenge (for the deaths of people we know) which can be very motivating.
This is all baseless speculation of course that I just came up with off the top of my head, but it sounds cool to me. It might run the risk of being like losing a sibling in DA2 which didn't give us enough time to really connect with them, but I hope that if something like this does happen (they want us to feel emotions at the lose of people in the veil tear) they give us a good junk of time to get to know the NPCs around the event before hand.